On February 21, 2008 at 14:16:02 UTC, a 6.0 earthquake occurred at 41.153 N, 114.867W, 10 km ENE of Wells, Nevada. This event caused significant damage in the historical section of the town. Unfortunately, there are no near-source strong motion recordings; however, at the time of the earthquake the EarthScope transportable array (TA) stations provided excellent records of the mainshock and aftershocks.

Figure 2.53:
Locations of the transportable array and ANSS (ELK) stations. Solid symbols identify stations used in the finite-source inversion. Station N12A was omitted since it experienced a non-linear response, and O11A was omitted since ELK provided information from the same azimuth.

Figure 2.54:
Slip model for SE-dipping fault plane. The color scale shows slip and the bars show the slip direction.

We determined a moment tensor solution using three-component, low frequency (0.02 to 0.05 Hz) displacement waves recorded by 47 stations of the NSF TA, and ANSS broadband seismic stations. Using the BSL seismic moment tensor code and the Song et al. (1996) velocity model, we obtained a normal faulting mechanism with the focal parameters strike=205, 35; rake=-96, -82; dip=40, 50. The source depth was constrained at 7 km as determined by the University of Nevada Reno Seismological Laboratory. The scalar seismic moment was determined to be dyne cm, corresponding to a 5.9. The normal mechanism is consistent with the trend of basin and range faulting in the region; however, there is some question about whether west-dipping or east-dipping faults are active in the region.

To determine the finite-source parameters we have inverted the three-component, broadband (f 0.02 Hz) displacement waveform data recorded at the 7 closest, on-scale, US and TA network stations (Figure 2.53) using the method of Kaverina et al (2002). We tested both nodal planes of the moment tensor double-couple solution over a range of rupture velocities, and found that the east-dipping nodal plane consistently provided the best fit to the data. Although the maximum in the goodness of fit parameter (variance reduction) is relatively broad, the best rupture velocity was found to be 2.8 km/s, or 78% of the shear wave velocity at the hypocenter depth. These initial inversions considered a constant rake (slip angle) obtained from the double-couple solution. We also performed an inversion allowing the rake to vary over the rupture plane, which resulted in a slightly more compact slip distribution. As Figure 2.54 shows the rupture is bilateral and slightly down-dip, but the largest slip is located to the southwest of the hypocenter in the direction of the town of Wells, Nevada. The slip in the variable rake model shows some variation but is predominantly normal, with the east-block down relative to the west-block. The peak slip in this model is 85 cm, with an average of 13 cm. The scalar seismic moment obtained by integrating the fault slip is
dyne cm, corresponding to 6.0. A rise time of 0.3 seconds was assumed. As Figure 2.55 shows, the fit to the regional data is very good.

Figure 2.55:
Observed three-component displacement records (black) are compared to synthetics (red). The data and synthetics are broadband with no filtering other than the polezero instrument response removal for the data.

Figure 2.56:
Simulated PGV (color shading) is compared to the USGS ShakeMap (contours) and PGV from TA stations (numbers in parenthases). There is good correlation between the finite-source simulated PGV and observations; however, the USGS ShakeMap shows values an order of magnitude larger. There were no near-fault strong motion data available for use in the USGS ShakeMap.

There are no strong motion stations located within 200 km of the Wells, NV earthquake, and, therefore, the ShakeMap (Wald et al., 1999a) is based solely on empirical ground motion relations and scaling from reported Community Internet Intensity Map values (Wald et al., 1999b). Using the slip distribution in Figure 2.54, and assuming a NEHRP class C site (555 m/s), we have simulated the near-fault strong shaking to produce a ShakeMap using the method proposed by Dreger and Kaverin (2000), and as discussed in Dreger et al.(2005) and Rolandone et al. (2006). Figure 2.56 compares the simulated peak ground velocity (shaded map) with the USGS ShakeMap (contours) and PGV at the regional TA stations. The simulated values are consistent with the observations, whereas the USGS ShakeMap over predicts values by more than a factor of 10. In Wells, NV we simulate a PGV of 10 cm/s, which is large enough to account for the considerable damage to the historic, unmaintained, unreinforced masonry buildings. The simulated sense of motion in Wells is down and to the east, which is consistent with reported westward chimney toppling, and sliding of heavy objects. This event occurred in a poorly instrumented region and demonstrates the difficulty in obtaining a ShakeMap under such conditions. This analysis shows, as in Dreger and Kaverina (2000), that in such poorly instrumented regions a regional data derived finite-source model from regional waveform modeling can be used to accurately simulate near-fault strong ground motions when no such recordings exist.